1. Field of the Invention
The present invention relates to wavelength and bandwidth calibration, and particularly for relative wavelength calibration, and for bandwidth calibration by convolution.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) will use the F2-laser operating around 157 nm.
Higher energy, higher stability, and higher efficiency excimer and molecular fluorine lasers are being developed as lithographic exposure tools for producing very small structures as chip manufacturing proceeds into the 0.18 micron regime and beyond. Specific characteristics of laser systems sought to be improved upon particularly for the lithography market include higher repetition rates, increased energy stability and dose control, increased percentage of system uptime, narrower output emission bandwidths, improved wavelength and bandwidth accuracy, and improved compatibility with stepper/scanner imaging systems.
Various components and tasks relating to today""s lithography laser systems are increasingly designed to be computer- or processor-controlled. The processors are programmed to receive various inputs from components within the laser system, and to signal those components and others to perform adjustments such as gas mixture replenishment, discharge voltage control, burst control, alignment of resonator optics for energy, linewidth or wavelength adjustments, among others.
It is important for their respective applications to the field of sub-quarter micron silicon processing that each of the above laser systems become capable of emitting a narrow spectral band of known bandwidth and around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, are being continuously improved upon. Depending on the laser application and imaging system for which the laser is to be used, line-selection and/or line-narrowing techniques are described at U.S. patent application Ser. Nos. 09/317,695, 09/317,527, 09/130,277, 09/244,554, 09/452,353, 09/602,184, 09/599,130 and 09/629,256, and U.S. Pat. Nos. 5,761,236, 6,081,542, 6,061,382 and 5,946,337, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018, and 4,829,536, all of which are hereby incorporated by reference. Some of the line selection and/or line narrowing techniques set forth in these patents and patent applications may be used in combination.
Techniques are also available for tuning and controlling central wavelengths of emission. Absolute wavelength calibration techniques use a known absorption or emission line around the wavelength of interest as a reference (see U.S. Pat. Nos. 4,905,243, 4,926,428, 5,450,207, 5,373,515, 5,978,391, 5,978,394 and 4,823,354, and F. Babin et al., Opt. Lett., v. 12, p. 486 (1987), and R. B. Green et al., Appl. Phys. Lett., v. 29, p. 727 (1976), as well as U.S. patent application Ser. Nos. 09/416,344 and 09/271,020 (each application being assigned to the same assignee as the present application), all of the above being hereby incorporated by reference).
Babin et al. discloses using the opto-galvanic effect to determine the KrF-laser absolute emission wavelength. A galvatron having an anode and a cathode is set in the optical path of the laser beam. An Fe vapor fills the galvatron. A voltage is monitored between the cathode and anode. The emission bandwidth of the laser is narrowed and the central wavelength tuned through a range around 248 nm. When the wavelength of the beam impinging the Fe-vapor filled gas volume between the cathode and the anode corresponds to an atomic transition of Fe, a resonance between the levels causes a marked change in voltage between the anode and cathode. Since the absorption lines of Fe are well known and consistent, e.g., based on standards set forth by NIST, the absolute wavelength of the narrowed laser emission band is determinable.
U.S. Pat. No. 4,823,354 to Znotins et al. describes using a photodetector to detect the intensity of light emitted from a KrF-laser. Znotins et al. disclose to use a galvatron having benzene vapor inside, whereas U.S. Pat. No. 5,450,207 to Fomenkov discloses the same technique instead having an Fe cathode inside. The cathode of Fomenkov gives off Fe vapor which fills the galvatron when a current is generated between the cathode and an associated anode. Light emitted from the KrF-laser traverses the gaseous benzene or iron medium of the galvatron before impinging the photodetector. When the wavelength corresponds to an atomic transition of the gas medium of the galvatron, the gas absorbs the light, and the intensity of light detected is reduced. Thus, the absolute wavelength of emission of the KrF-laser is also determinable in this alternative way.
Another known technique uses sealed hollow cathode lamps containing Fe-vapor in a Ne-buffer gas environment. See Hammamatsu Datasheet: Opto-Galvanic Sensor, Galvatron L 2783 Series, November 89, Japan. Thus, the Fe-lamp has become an important and reliable measuring tool for absolute wavelength calibration for KrF-lithography laser systems in the 248 nm spectral region. The ""344 application and ""391 and ""394 patents, mentioned above, describe techniques for absolute wavelength calibration for ArF and F2 lasers.
The ""243 patent, also mentioned above, describes the use of a monitor Fabry-Perot etalon to determine relative wavelength shifts away from the known Fe absorption lines, e.g., at 248.3271 nm and 248.4185 nm, among others. To do this, the laser wavelength is first calibrated to the absolute wavelength reference line, e.g., 248.3271 nm, and the laser beam is directed through the etalon. An interferometric image is projected onto a solid state image detector such as a CCD array. Next, the laser wavelength is tuned away from the 248.3271 nm line to a new wavelength. A new image is projected onto the detector, and a comparison with the original image reveals the new wavelength because the free spectral range (FSR) of the monitor etalon is presumably known (e.g., 9.25 pm). For example, if it is desired to tune the laser to 248.3641 nm, then the wavelength would be adjusted 37 pm above the 248.3271 nm Fe vapor absorption line to exactly coincide with four FSRs of the monitor etalon.
A mercury lamp for emitting reference light of known wavelength is used in U.S. Pat. No. 5,748,316. The reference light and the laser beam are each directed to the monitor etalon. A comparison of the fringe patterns produced by the reference light and the laser beam allows a determination of the wavelength of the laser beam relative to that of the reference light.
The demands of laser systems today require very specific determinations of the wavelength shift. Thus, a more precise technique is desired for calibrating the relative wavelength shift.
Other optical characteristics of a laser beam that are desired to know and control are the bandwidth and spectral purity. The bandwidth can be measured as the full width at half maximum (FWHM) of a spectral intensity distribution of a measured laser pulse. The spectral purity is determined as the spectral range within which lies 95% of the energy of the laser pulse.
The bandwidth of a radiation source used, e.g., in photolithographic applications, is constrained by its effect on imaging resolution due to chromatic aberrations in optics of catadioptric imaging systems. The bandwidth of a laser beam can be determined from measuring the widths of fringes produced as the laser beam is passed through a monitor etalon and projected onto a CCD array. A grating spectrometer may also be used and the bandwidth measured in a similar fashion (see U.S. Pat. Nos. 5,081,635 and 4,975,919, each of which is hereby incorporated by reference). It is desired, however, to have a technique for more precisely determining the bandwidth of a laser beam.
It is a first object of the invention to provide a precise technique for precisely adjusting a laser beam to a desired wavelength shifted from a known wavelength.
It is a second object of the invention to provide a precise technique for monitoring the absolute bandwidth or spectral purity of a laser beam.
In accord with the first object, a method is provided for determining the relative wavelength shift of a laser beam away from a known reference line, such as an absorption line of a gas in an opto-galvanic cell or a reference line of a reference laser. A wavemeter is used, and a monitor etalon is preferably used as the preferred wavemeter device, wherein the FSR of the etalon used to calculate the wavelength shift is determined based on a calculated gap spacing between the etalon plates. The gap spacing is determined based on a fit to measured values of wavelength deviations of the FSR as a function of the relative wavelength shift. The FSR used to calculate the wavelength shift may also be based on the wavelength shift itself. Thus, the wavelength shift of the laser beam is calculated as the number of FSRs counted as the wavelength is tuned from the known reference line, wherein the value of the FSR used in the calculation for each fringe crossed as the wavelength is tuned is calculated based on the calculated gap spacing, and preferably the wavelength shift itself.
In accord with the second object, a method is provided for measuring the absolute bandwidth of a tunable laser beam using an opto-galvanic or absorption cell. The laser beam is directed to interact with a gas in the cell that undergoes an optical transition within the spectral tuning range of the laser. The beam is tuned through the optical transition line of the gas in the cell, and the opto-galvanic or absorption spectrum of the line is measured. The measured bandwidth is convoluted or broadened by the bandwidth of the laser beam used in the measurement. The bandwidth or spectral purity of the laser beam is determined based on the width of the measured spectrum and a known correspondence between this measured convoluted width and the bandwidth of the laser beam.